Metal-free photoanodes for C–H functionalization

Organic semiconductors, such as carbon nitride, when employed as powders, show attractive photocatalytic properties, but their photoelectrochemical performance suffers from low charge transport capability, charge carrier recombination, and self-oxidation. High film-substrate affinity and well-designed heterojunction structures may address these issues, achieved through advanced film generation techniques. Here, we introduce a spin coating pretreatment of a conductive substrate with a multipurpose polymer and a supramolecular precursor, followed by chemical vapor deposition for the synthesis of dual-layer carbon nitride photoelectrodes. These photoelectrodes are composed of a porous microtubular top layer and an interlayer between the porous film and the conductive substrate. The polymer improves the polymerization degree of carbon nitride and introduces C-C bonds to increase its electrical conductivity. These carbon nitride photoelectrodes exhibit state-of-the-art photoelectrochemical performance and achieve high yield in C-H functionalization. This carbon nitride photoelectrode synthesis strategy may be readily adapted to other reported processes to optimize their performance.


References
Fig. S1 Photoelectrode preparation through a spin-coating pre-treatment and CVD process.In the pre-treatment step, 50 mg melamine-cyanuric acid (MCA) supramolecular mixture and 50 mg polymer were dissolved or dispersed in 500 µL solvent.The prepared solution was spin-coated onto the FTO substrate to get a homogeneous precursor-polymer composite film, which will form the porous top layer in the dual-layer structure later.Specifically, S-LEC and PS are dissolved in DCM, while PVP and PEG are dissolved in water.Then, 5 g melamine were placed in a 29.5 mL rectangular alumina crucible.The FTO glass with the supramolecules and polymer complex films is placed on the top of the crucible.During the CVD process, the polymer in the composite film decomposed, resulting in the porous structure of the top layer.Melamine vapor passes through the top layer and condenses and polymerizes on the surface of the substrate, which gives the bottom layer in the dual-layer structure.Fig. S8 UV-visible absorption spectra of DCN and blade-coated CN film (inset).After preparation of the DCN electrode by CVD, the remaining CN powder in the crucible was mixed with ethylene glycol (10 mg/ml).The mixture was ground and the suspension was deposited onto an FTO substrate by doctor blading (see inset).The film was dried on a hot plate at 70 °C, yielding a strongly scattering film.UV-vis absorption measurements were performed in transmittance mode using a PG Instruments TG70+ UV/vis spectrometer.Supplementary Discussion 1 Excitation of the DCN photoanode by light converts carbon nitride into an excited state.The bias voltage of +0.22 V facilitates the separation of charges by extracting electrons.Photogenerated holes oxidize N-aryl-tetrahydroisoquinoline to the corresponding iminium cation (2e -/H + process), which upon nucleophilic attack of HO -is converted into the product.Iminium cations are ubiquitous intermediates in photoredox catalysis and were also postulated in photocatalytic functionalization of N-aryl-tetrahydroisoquinolines by carbon nitrides. 2,3 ue to the constant bias potential of only +0.22 V vs. Fc+/Fc, compared to a constant current of 5 mA, 4 the aminoalcohol 2 is selectively obtained instead of the amides.HO -species are replenished upon water reduction at the Pt cathode.Although the scope of the reaction was investigated in methanol, screening of reaction conditions revealed that the yield of 2a was high (86%), when the reaction was performed in wet acetonitrile or acetone (Table 1, entries 1,2).Compound 2 has been reported and characterized previously. 5

Fig. S2
Fig. S2 Surface roughness analysis of the films prepared by spin coating, dip casting, and doctor blading.Sq is the Root-mean-square height, which is obtained by the following equation:

Fig. S4
Fig. S4 Photoelectrodes prepared with different concentration of polymer (a, c) 3 mmol/l, (e) 6 mmol/l, (b, d) 12 mmol/l, and their topographies are measured by both white light interferometry and cross-section SEM, which give consistent results.(e) Topography and line profile of a photoelectrode with 6 mmol/l polymer, measured by white light interferometry.The film thickness is about 1.8 µm.

Fig. S6
Fig. S6 Fourier transform infrared spectra of the precursor-polymer mixture, CN, and DCN electrodes.

Fig. S9
Fig. S9 Steady-state fluorescence spectrum of a DCN electrode acquired using a picosecond laser diode (excitation at 375 nm) in a PicoQuant TCSPC FluoTime 250 spectrometer operating at steady-state mode.

Fig. S12
Fig. S12 The stability of DCN electrodes was measured under the conditions for PEC reactions (+0.22 V vs. Fc + /Fc, 0.1 M LiClO4 in methanol).SEM images of the DCN electrodes (b) before and (c) after the stability test.

Fig. S16 1 H
Fig.S161 H NMR of the THIQ substrate and product before and after isolation.See literature.5

5
Fig.S161 H NMR of the THIQ substrate and product before and after isolation.See literature.5

Table S1 .
DCN trace metal analysis by inductively coupled plasma OES

Table S2 .
Current density of different DCN electrodes in water

Table S3 .
Current density of different DCN electrodes in methanol

Table S1 .
DCN trace metal analysis by inductively coupled plasma optical emission spectroscopy.DCN was either synthesized directly on float glass and simply peeled off or mechanically scratched off from the FTO substrate.Polystyrene (PS) was used as comparison.The elevated Sn concentration (~0.5 %) in the DCN on FTO sample was caused by tight adhesion of DCN to the FTO, requiring vigorous mechanical scratching with steel tweezers for removal.Most notable contaminations of DCN on glass were ~0.02 % of copper (Cu), zinc (Zn), potassium (K), and calcium (Ca).Sodium (Na) is typically elevated.